Dissolved oxygen and suspended particles regulate the benthic flux of iron from continental margins William B. Homoky a, ⁎, Silke Severmann b , James McManus c , William M. Berelson d , Timothy E. Riedel d , Peter J. Statham a , Rachel A. Mills a a University of Southampton, Ocean and Earth Science, National Oceanography Centre Southampton, SO14 3ZH, UK b Institute of Marine & Coastal Science, Rutgers, The State University of New Jersey, 71 Dudley Road, New Brunswick, NJ 08901-8521 USA c Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331-5503, USA d University of Southern California, Department of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089-0740, USA abstract article info Article history: Received 9 October 2011 Received in revised form 8 March 2012 Accepted 22 March 2012 Available online 1 April 2012 Keywords: Iron Oxygen Shelf sediment Pore water Resuspension Particles Benthic flux Incubation We present ex situ sediment incubation results from the California and Oregon shelves and compare the calculated benthic flux of dissolved Fe with those from in situ incubations and pore water concentration profiles. We also examine the influence of oxygen depletion and sediment re-suspension on benthic Fe exchange. Ex situ incubation of the California and Oregon shelf sites yielded average benthic Fe fluxes of 3.5 and 8.3 μmol m -2 day -1 , respectively, compared to 17 and 55 μmol m -2 day -1 from the in situ Lander determinations, and 73 and 103 μmol m -2 day -1 from modeling of pore water concentration profiles. Differences between benthic Fe flux estimates are primarily accounted for by [1] differences in Fe (II) oxidation kinetics, which result from distinct oxygen consumption rates between incubation methods, and the absence of kinetic considerations in the overlying bottom water in pore water flux calculations, and [2] the effects of biological sediment irrigation that are best represented by in situ incubations due to their sampling area and preservation of bottom water conditions. Bottom water oxygen concentrations were higher at the California shelf site than that at the Oregon shelf site, and probably accounted for the greater discrepancy between methods used to determine benthic Fe flux. The comparison of techniques used to determine benthic Fe flux indicates that the concentration of bottom water oxygen exerts a principle control over the fate of dissolved Fe entering the overlying bottom water — supporting the view that the expansion of coastal hypoxia has the potential to enhance the benthic supply of Fe (II) to shelf waters. An episode of surface sediment re-suspension during ex situ incubation led to a rapid removal of 76–89% of dissolved Fe from seawater, followed by a steady return towards initial seawater concentrations during particle settling, indicating that diffusive inputs of dissolved Fe from sediment pore water are rapidly adsorbed and desorbed by particles during periods of benthic re-suspension. The findings suggest that dissolved Fe concentrations in bottom waters may reflect an equilibrium concentration of non-stabilized aqueous Fe and particle-adsorbed Fe phases — where the addition of suspended particles to bottom waters leads to scavenging of dissolved Fe into labile particulate Fe phases. Thus we suggest that suspended particles are a significant buffer of dissolved Fe released from shelf sediments, an important transport mechanism for benthic Fe inputs, and a regulator of dissolved Fe concentrations in seawater. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The supply and distribution of dissolved iron (dFe) is known to impact the extent of primary production (e.g., Coale et al., 1996; Martin and Fitzwater, 1988; Martin et al., 1989) and eco-system structure in some regions of the ocean (Hutchins et al., 1999; Sunda and Huntsman, 1995). The concentration of dFe in the ocean interior is characteristically low relative to coastal environments, where shelf sediments sustain an input of dFe to seawater via ferruginous sediment diagenesis (Elrod et al., 2004; Lohan and Bruland, 2008; Raiswell and Anderson, 2005; Severmann et al., 2010). Due to the oxidizing potential of modern seawater, dFe in coastal zones is inefficiently transferred to the open ocean. Labile particulate Fe (LPFe) phases are also known to be an important source of Fe for marine phytoplankton in coastal zones (Bruland et al., 2001; Chase et al., 2005; Croot and Hunter, 1998, 2000; Johnson et al., 1999), and studies indicate that LPFe may be transported hundreds of kilometers from coastal zones to remote Fe-depleted surface ocean settings (Lam and Bishop, 2008; Lam et al., 2006; Nishioka et al., 2007; Planquette et al., 2011; Siedlecki et al., 2012; Slemons et al., 2010). Marine Chemistry 134-135 (2012) 59–70 ⁎ Corresponding author. Tel.: + 44 2380 599 346. E-mail address: wbh@noc.soton.ac.uk (W.B. Homoky). 0304-4203/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2012.03.003 Contents lists available at SciVerse ScienceDirect Marine Chemistry journal homepage: www.elsevier.com/locate/marchem